The present invention relates to a home-use video tape recorder (VTR), and more particularly to a video tape recorder having a function for detecting a noise-bar period generated in a special reproduction mode, called "high-speed picture search" for reproducing an image at high speed, so as to control various circuits for processing reproduced signals.
A home-use VTR for recording/reproducing an NTSC television signal is structured such that a brightness signal component in a video signal is converted into a low-frequency carrier wave FM signal having a low-frequency carrier wave. Moreover, a color signal component of the video signal is converted into a low-frequency color signal having a low frequency, for example, 629 KHz which is lower than the FM frequency of the brightness signal. The converted low-frequency color signal is superimposed on the low-frequency carrier wave FM signal so as to be magnetically recorded on a video tape. In this case, two video heads provided for a rotating cylinder are operated to record the foregoing superimposed signals on two adjacent video tracks of the video tape. As a recording method, for example, a guard band-less recording method is employed in order to raise the recording density.
FIG. 10 is a diagram showing a portion of a brightness signal reproducing system of a conventional VTR.
Signals respectively reproduced by two video heads 50a and 50b are amplified by front amplifiers 51 and 52, and then alternately selected by a switcher circuit 53 so as to be formed into a continuous signal.
An output signal from the switcher circuit 53 is supplied to a HPF (High-Pass Filter) 54 so that a color signal component (a low-frequency converted color signal) contained in the output signal is removed and a brightness signal component (a low-frequency carrier wave FM signal) is extracted. Drop out of the low-frequency carrier wave FM signal is compensated by a DOC (Drop-Out Compensator) 55.
When the DOC 55 has detected drop out generated in the reproduced signal, the DOC 55 substitutes a low-frequency carrier wave FM signal forward by 1 H (horizontal scanning period) for the drop out portion by using line correlation of a video image.
The level change of an output signal from the DOC 55 is limited by a limiter circuit 56, and then the output signal is FM-demodulated by a FM demodulation circuit 57 so that the brightness signal component is extracted. The characteristic of the brightness signal component is corrected by a de-emphasis circuit 58, and then the carrier wave component of the FM signal is removed by a low-pass filter (LPF) 59. Moreover, the transference time is aligned with that of the color signal reproducing system, and then the brightness signal component is supplied to an output amplifier 60.
A usual VTR is structured to have a special reproduction mode, such as a still-image reproduction mode and a high-speed picture search (speed search) mode, in which the reproducing video head traces a plurality of video tracks on the video tape as it traverses the video tracks. Therefore, reproduced outputs from the plural video tracks are alternately output, thus causing constriction to be generated in the reproduced FM signal. As a result, the amplitude of the reproduced FM signal is reduced and, therefore, the S/N ratio deteriorates. Thus, bar-shape noise, called noise bars or noise bands, is generated on the displayed image.
As a countermeasure to remove the noise bar, drop out compensation has been employed with which the period, in which the envelope of the reproduced FM signal has been reduced, is replaced by a reproduced FM signal forward by 1 H. However, the drop out compensation has the following problems:
FIGS. 11A to 11D are graphs showing examples of the actual noise bar period, the envelope waveform of the reproduced FM signal, the drop-out detection output waveform from the DOC and the input waveform to the clamp circuit.
(1) The drop out compensation must be performed quickly. In general, delay time caused from detection of the drop out is set to be not longer than 1 .mu.s and delay time involved to return to the usual operation mode is set to be not longer than several .mu.s.
On the other hand, the reproduced FM signal is, as shown in FIG. 11A, irregularly changed in the noise bar period because the portion of the noise component is enlarged. Therefore, a signal for compensating the drop out is output from the DOC in the noise bar period, the signal being, as shown in FIG. 11B, composed of a plurality of pulses which are randomly output. That is, the conventional DOC is not structured to output one pulse which includes the noise-bar period.
(2) The reproduced FM signal or the brightness signal obtained by FM-demodulating the reproduced FM signal, each of which has been obtained in the noise-bar period is not a normal signal. Thus, the operations of the circuits in the reproducing system are adversely affected.
For example, a clamp circuit for clamping the level of a synchronizing signal added to the brightness signal, as shown in FIG. 11C, unintentionally clamps the brightness signal component with a lower level of the noise waveform if a noise waveform, the potential of which is made to be lower than the leading end (sync. tip) of the synchronizing signal, is included in the brightness signal component, as shown in FIG. 11D. Thus, there arises a problem in that, for example, the clamp potential of the synchronizing signal is disordered.
An automatic gain control (AGC) circuit for automatically adjusting the gain of an amplifier while monitoring the level of the reproduced signal involves unintentional shift of the optimum adjustment point attributable to an abnormal signal generated in the noise-bar period. Therefore, when the noise-bar period has been ended and the mode is returned to the normal operation mode, an excessively long time is required to return to the optimum adjustment point.
The demodulation sensitivity of the FM demodulation circuit and the like is controlled by a determination output from a mode determination circuit for determining whether the mode is the SVHS mode or the NVHS mode. However, the determination output from the mode determination circuit in the noise-bar period is not a reliable output. Therefore, the FM demodulation circuit cannot normally be controlled.
If noise having a large amplitude is generated in the noise-bar period, there sometimes arises a problem in that generated noise exceeds the dynamic range of a circuit in the rear of the circuit in which noise above has been generated.
Although the reproducing system of the conventional VTR is arranged to compensate drop out in accordance with a drop-out detection output in the noise-bar period, it suffers from a problem in that the operations of the circuits in the reproducing system are adversely affected because the reproduced FM signal or a brightness signal obtained by FM-demodulating the reproduced FM signal is not a normal signal.